Plug valves are typically known to place high forces on the actuator. This is due to fluid pressure acting on the end face of the plug. Some valves will incorporate a passage that is drilled from the nose of the plug to an internal cavity. Pressure in this internal cavity acts on a shoulder on the valve between the plug portion and the stem portion to force the plug in the downward direction, partially counteracting the forces acting on the plug end face that push the valve upward. The working area of this shoulder is equal to the working area of the plug face minus the area of the valve stem. Therefore, the working area occupied by the valve stem prevents complete pressure balancing across the valve and only provides for partial balancing, which in turn requires higher actuator force to achieve performance requirements and/or results in lower dynamic response. Creating a very small valve stem can reduce the actuator force required. However, the stem must be large enough to transmit the required actuator loads with margin.
In applications that require quick response, plug valves have historically been actuated by hydraulic actuators. Hydraulic actuation provides a high power density that is not achievable with electric actuation. Along with hydraulic actuation, however, one also unfortunately gets oil leaks, fire concerns, filtering requirements, oil piping, and a costly hydraulic power unit. Such hydraulic power units may cost in excess of $50,000 in certain gas turbine applications.
Electric actuation has typically been reserved for low power applications: high force at low speeds or low force at high speeds. However, a large number of applications do not fall in these categories as end users define valve operating pressures and dynamic response requirements, which in turn requires the more costly hydraulic actuation. Reduction of plug valve actuation forces is necessary to economically applying electric actuators on high performance applications, such as valves for turbine fuel metering. Accordingly, there is a desire and need to reduce valve actuation force and/or increase dynamic valve response in the art.